Today, thermal ink-jet printers have gained wide acceptance for use in printing computer images due to their small size, portability, efficiency, and ability to produce high quality print. The print head of a typical thermal ink-jet printer has a plurality of precisely-formed nozzles, each nozzle being in fluid communication with a chamber that receives ink from an ink reservoir. Each chamber is adjacent to an electrical resistance element, known as a thermal ink-jet resistor, which is located opposite the nozzle so that ink can collect between the nozzle and the resistor. Electric printing pulses heat the thermal ink-jet resistor, causing a small portion of the ink adjacent to the resistor to become vaporized, thereby imparting mechanical energy to a quantity of the ink and propelling the ink through the nozzle of the print head toward a print medium. The ejected drops collect on the medium and form printed characters and/or images thereon. The printing is generally accomplished by incrementally moving the medium in a first direction relative to the print head and moving the print head in a second direction which is perpendicular to the first direction. A number of nozzles may be supplied across the print head so that a number of droplets may be fired from the print head at once. The spacing of the nozzles and the incremental stepping of the print head and the medium define the resolution of the printed image. A printer with 600 dots per inch resolution can print 600 dots per square inch or, in other words, each dot printed from the print head can cover approximately 1/600th.times.1/600th square inch area. This small area which may be covered by one dot is generally referred to as a picture element, or "pixel".
Halftone printing is a type of printing that can be used to print text but is particularly useful for printing graphical images, such as in desktop publishing, so that more detailed images can be created. The method takes advantage of the tendency of the human eye to blur together groups of dots and merge them into a single perceived shade. The more dots per unit area, the darker the shade will appear.
Typically, such printing entails dividing the source image into a number of pixels which corresponds to the resolution of the printer, and then assigning each pixel a halftone level, depending on the shade of gray of the particular pixel from the source image. The shade falls within a gray scale which is a progressive series of shades, ranging from black through white. The number of shades of gray which can be used to describe each pixel in the source image (and thus the number of halftone levels) depends upon the amount of memory allocated for each pixel to be printed. The more bits used in coding shades of gray, the more gradations which become possible. For example, the use of two bits per pixel allows for four halftone levels per pixel, six bits allows 64 levels, and 8 bits allows 256 levels. However, as the number of bits increases, so does the need for storage capacity. For example, with 256 shades of gray, one byte of storage is required for each pixel in the image. Accordingly, a small image 100 pixels wide by 100 pixels high requires 10,000 bytes of storage. As a result, detail and storage requirements are usually balanced to provide the best possible image at the least cost in storage capacity.
The halftone level of each pixel corresponds to a probability of printing a dot at that particular pixel. For example, in one embodiment, a halftone value of zero means that a black dot absolutely should not be printed at that pixel, a value of 255 means that a dot absolutely should be printed at that pixel, and a value of 128 means that there would be slightly more than a 50% chance (128/255) that a dot would be printed at that particular pixel. Thus, for example, a source image which consists of an entire page colored a "medium" gray color would typically be mapped such that all the pixels to be printed on the printed medium are assigned a halftone level of 128, thereby creating a 50% chance at each pixel that a dot will be printed. Thus, on that page, dots will be placed at approximately 50% of the available pixels. When printed, the dots visually blur together to appear as a "medium" shade of gray.
However, the quality and consistency of the halftone images printed by ink-jet printer is affected by the temperature of the print head. As the temperature of the print head rises, the drop size produced by each nozzle increases, because the rise in temperature lowers the viscosity of the ink in the chamber, causing larger vapor bubbles to be produced, thereby ejecting more massive droplets. Accordingly, the larger droplets make bigger spots on the page upon which the printer is printing, and these larger spots, in turn, cause the image that is produced to look different than the image that is produced when the print head is operating at a lower temperature. Moreover, more massive droplets mean that more ink is being deposited on the page, increasing the chances that paper cockling and ink smearing may occur.
The temperature of the print head rises most significantly when a very dark image is being printed, because numerous dots are being ejected from the print head to produce the dark image, and numerous resistors are being heated to produce the dots, the accumulated heat of the resistors raising the temperature of the print head. Thus, as the print head finishes a line of print, or "swath", the print head will often be at a higher temperature than when it began the swath. Also, the print head may remain at this higher temperature during the next swath to be printed. For example, a page that is intended to be printed as a uniform shade of gray may turn out with variations in the shade of gray produced at various locations on the page. If the print head prints from left to right, and from top to bottom, the right part of the page may be at a darker shade of gray than the left part of the page and, similarly, the bottom half of the page may be a darker shade of gray than the top half of the page. Moreover, if the print head temperature during actual printing is different than the print head temperature assumed in generating the halftone levels, then the shades of the printed image will not correspond well with the source image data.
Previously, the problem of poor print quality due to temperature variations has been addressed by controlling the temperature of the print head. However, because active cooling, such as through the use of thermo-electrical cooling, circulation pumps, and the like, is not cost effective, passive cooling has been the method utilized by most thermal ink-jet manufacturers. One method of passive cooling is to utilize an aluminum heat sink to absorb the heat generated by the print head. In utilizing this method, after each print line, the micro code of the printer will command the print head to wait in the margin until the print head cools to the desired temperature range through dissipation of heat to the heat sink.
However, passive cooling of the print head is not without disadvantages. The additive affect of the delay while the print head cools can become quite noticeable over time and have a substantial impact on the throughput of the printer. In addition, if the print head is not cooled to nearly the same temperature prior to each print line, the print head will produce a variable droplet size, and therefore a variable output, from swath to swath. Such swath to swath variations are extremely problematic because they are the most visible. Moreover, both passive and active temperature control methods require the use of a temperature sensor on the print head, which can wear out and add to the cost of the printer. In addition, for temperature control schemes to be successful, the print head temperature must be maintained within about a 5C range, and maintaining such control while allowing the print head input power to roam freely for all densities of print can be difficult and expensive.
Another approach to the problem of temperature variations has been to vary the voltage and pulse width applied to the resistance elements that cause the droplets to be formed and ejected. For example, U.S. Pat. No. 5,483,265, issued to Kneezel et al., discloses a control method in which the temperature of the print head of the ink-jet printer is sensed and the heater element 26 is energized with a pulse of predetermined power and duration based on the sensed temperature, such that the resulting spot size is near the optimum size. According to the patent, after the print head temperature is sensed, a predetermined pulse duration and voltage for the sensed print head temperature can be retrieved from a lookup table and then applied to the heater element. If the sensed temperature is greater than a predetermined temperature for the desired ink droplets size, the pulse duration is shortened and the pulse voltage is increased to maintain the desired droplet size. However, if the sensed temperature is less than the predetermined temperature, the pulse duration can be lengthened and the voltage can be decreased to maintain the desired size.
U.S. Pat. No. 5,610,638 issued to Courtney, discloses another approach to the compensating for temperature induced droplet size variations. According to this patent, a temperature sensor 16 senses the temperature adjacent to the print head and selects either a single-pass 100% coverage print mode or a double-pass 50% coverage checker board print mode for printing. In the single-pass mode, each swath of printing is printed in one pass, while in the double-pass mode two passes of the print head are used for each swath of printing. The patent also discloses varying the injection rate (or frequency) based upon the sensed temperature. As shown in FIG. 3 of the patent, when the temperature sensed is less than or equal to 30? and the density of the image to be printed is low, the ejection frequency is selected to be 6 khz and the single-pass mode is selected. However, if the image to be printed is determined to be of high density and the temperature is less than or equal to 30?, the frequency is reduced to 4.5 khz and the single-pass mode is maintained. When the temperature is greater than 30? for both low density and high density images, the frequency is kept at 6 khz and the printer utilizes the double-pass mode.
However, the temperature compensation means disclosed by these patents have certain disadvantages. First, both require a temperature sensor to be used to determine the temperature of the print head. Also, adjusting the energy delivered to the nozzles by varying the voltage or pulse width, as disclosed in Kneezel, can reduce print head life, and is ineffective beyond a certain energy level. In addition, while controlling energy levels delivered to the print head may work well for certain types of printers, it may be difficult to implement in ink-jet printers because the mechanics of bubble generation in a thin film of liquid ink make it difficult to precisely control droplet mass by controlling firing energy. Moreover, adjusting the ejection frequency, as disclosed in Courtney, relies on firing droplets at a cycle time that is near the refill time of the print head thereby ejecting larger or smaller droplets. However, unless the manufacturing of the print head and ink cartridge are tightly controlled, the refill time of the printer can vary significantly, thereby causing large differences in refill dynamics and making the control method unpredictable. Neither patent discloses a temperature compensation scheme for halftone printing which involves the control of the number of dots ejected by adjusting halftone levels of pixels based upon temperature, and neither patent discloses a temperature prediction method which counts the number of dots to be fired by the print head to predict the print head temperature.
Accordingly, to overcome the above and other problems, it is desirable to have a system and method for compensating for temperature induced variations of droplet size in a thermal ink-jet printer which does not require tight control of the print head temperature, which does not require adjustment of the print head parameters, and which does not require temperature sensing means. Further, it is desirable to have such a system and method which effectively reduces swath to swath variations in print quality, as well as reduces paper cockling and smearing and increases reservoir lifetime.